The present invention relates generally to the manufacture of fiber-reinforced thermoplastic polymeric structural components and, more particularly, to an apparatus and method for single step, in-line compounding, deposition and compression molding of fiber-reinforced thermoplastic polymeric structural components.
Fiber-reinforced thermoplastic polymer structural components are most commonly manufactured from long fiber thermoplastic (LFT) granulates (pellets), glass mat thermoplastic (GMT) sheets, or pultruded sections. Long fiber-reinforced granulates typically consist of glass fiber bundles encapsulated with a thermoplastic through a cable coating or a pultrusion process. LFT granulates can be injection molded but are more commonly extrusion compression molded in order to preserve fiber length in the finished product. Although the damage to LFT granulates during processing is reduced when extrusion compression molded, some damage still occurs during the plastication process due to shear heating.
GMT sheets consist of a needle-punched glass mat impregnated with a thermoplastic polymer (typically polypropylene) to form a glass-reinforced thermoplastic sheet which is subsequently heated and compressed in a vertical compression press to obtain the final part shape. Desired mechanical properties of parts produced from GMT sheets can be custom tailored via the orientation of the glass fibers within the sheet. Overall mechanical properties are as good and many times improved over parts produced from LFT granulates, particularly in the area of impact strength. However, GMT sheets require preheating prior to compression molding and have flow limitations in the direction perpendicular to a die draw.
The pultrusion process is predominantly used in applications where the structural component requires optimal mechanical properties in one direction. Pultrusion typically involves impregnating fiber bundles with a polymer melt while the bundles are passed through a cross-head extrusion die, which also serves to shape the impregnated fibers into a predetermined section. Upon exiting the die, the polymer-impregnated fiber bundles are drawn into a cooling trough and cut to length upon exiting a haul-off unit. If it is desired to reshape these sections, as in compression molding, the sections must be reheated to the point where flow will occur under pressure. Also, the reheated sections require hand lay-up within the mold to obtain the desired fiber orientation.
Polymer components reinforced with fibers may also be manufactured using continuous in-line extrusion methods known in the art. Such methods involve the plastication of a polymer in a first single screw extruder from which the output is fed to a second single screw extruder. Fibers are introduced in the polymer melt in the second extruder either in chopped-segmented form or as continuous strands under a predetermined tension. The fiber-reinforced polymer compound is fed into an accumulator and then applied automatically or in a separate step to a compression molding tool wherein the fiber-reinforced polymer compound is shaped as required for a particular application. Alternatively, the fiber-reinforced polymer compound may be continuously extruded onto a conveyor and sectioned thereupon. The conveyor delivers the sectioned fiber-reinforced polymer compound to a placement assembly which removes the sectioned compound from the conveyor and places the compound upon the compression molding tool.
In-line extrusion methods used in the art to manufacture fiber-reinforced polymer compounds often damage the fibers during processing thus degrading the performance of the final reinforced composite structural component. Introducing fiber into the polymer melt within the extruder exposes the fiber to an extruder screw therein which rotates to create the polymer melt, mix the melt with the fibers, and move the resulting compound toward an outlet of the extruder. The rotation of the screw exerts shear forces upon the fiber resulting in wearing and eventually severance of the fiber. The forces within the extruder may also have an adverse effect upon the screw and the interior of the extruder barrel resulting in increased maintenance and cost. Additionally, the fiber may easily become tangled or otherwise mis-distributed within the extruder thus preventing a uniform distribution of the fiber throughout the polymer compound and resulting in an inconsistent disposition of individual fiber lengths.
Furthermore, the fibers within the extruder are exposed to the heat of the polymer melt for a substantial amount of time as the screw moves the fiber-reinforced polymer compound the length of the extruder. The temperature within the extruder can be, for example, in excess of three hundred and fifty degrees Fahrenheit. Natural fibers, which are lower in cost than synthetic fibers and are preferred for their recyclable properties, do not survive exposure to the magnitude of heat present within the extruder and thus tend to complicate the discussed extrusion methods of manufacturing fiber-reinforced polymer structural components discussed above.
Typical methods of extrusion manufacturing of fiber-reinforced polymer structural components do not permit the percentage of reinforcement fibers within the reinforced polymer compound to be varied during compounding or extrusion deposition. Various uses of fiber-reinforced polymer structural components may benefit from a controlled variation of fiber content within the reinforced polymer compound and hence throughout the resulting structural component. For instance, a portion of a particular structural component may require extra reinforcement whereas another portion of the same structural component may require little to no fiber reinforcement. Additionally, the structural component may call for various cross-weavings of the reinforcement fibers. Known extrusion methods allow neither variation of the percentage of fiber throughout the structural component during extrusion and deposition nor variation of the positioning of the fiber, i.e., cross-weaving, as applied to the reinforced structural component upon the compression mold thus limiting the effectiveness of such methods.
An in-line compounding and extrusion deposition compression molding apparatus for producing a fiber-reinforced molded structural component is provided. The apparatus allows a single step process for forming a polymer melt, extruding the polymer melt through a die channel, compounding the polymer melt in the die channel with at least one reinforcing fiber to form a fiber-reinforced polymer compound, depositing the fiber-reinforced polymer compound onto a compression mold, and molding the reinforced structural component therein.
In a preferred embodiment of the present invention the apparatus comprises a barrel having a body and an internal cavity formed therein. An extruder screw is rotatably disposed within the internal cavity to facilitate extrusion of a polymer melt which is also disposed within the internal cavity. The polymer melt is maintained at a predetermined temperature within the internal cavity of the extruder by the shear frictional forces of the rotating extruder screw and by a temperature mechanism disposed in the barrel. A deposition die head is disposed on a first end of the barrel for receiving the extruded polymer melt from the barrel. The deposition die head includes a die channel with a first opening proximate the barrel, connectively related to the internal cavity, and a second opening distal the barrel. The deposition die head may be releasably mounted to the barrel and the deposition die head, itself, may be comprised of a plurality of releasably mounted parts to facilitate operator access to the die channel. The apparatus further includes at least one fiber element for feeding at least one reinforcing fiber into the die channel of the deposition die head to form a fiber-reinforced polymer compound which is released from the second opening of the deposition die head onto a cavity of an open compression mold. The compression mold closes to form the fiber-reinforced molded structural component.
The apparatus, in a preferred embodiment, is movably disposed such that the apparatus may be maneuvered within the open compression mold in three dimensions, commonly understood to be the x, y, and z coordinate planes. The ambulatory nature of the apparatus allows disposition of the fiber-reinforced polymer compound in various concentrations and arrangements throughout the compression mold cavity. Thus, the amount of fiber reinforcement may be varied within the cavity of the compression mold resulting in a polymer structural component having enhanced reinforcement where desired.
The percentage of fiber within the reinforced polymer compound may also be varied through a simple adjustment of the deposition die head. The deposition die head may be fitted with a die lip which includes a deposition opening through which the fiber-reinforced polymer compound is passed during deposition thereof onto the cavity of the compression molding tool. Utilizing a die lip with a smaller opening allows less of the polymer melt to pass through the deposition opening thus increasing the percentage of reinforcing fibers relative to the volume of polymer melt. Contrariwise, a die lip with a larger opening will produce a fiber-reinforced polymer compound with a lesser percentage of reinforcing fiber relative to the volume of polymer melt.
The present invention further allows the percentage of reinforcing fibers within the fiber-reinforced compound to be varied by introducing additional reinforcing fibers or terminating existing reinforcing fibers mid-stream during formation and deposition of the fiber-reinforced polymer compound. In other words, the number of reinforcing fibers present in the fiber-reinforced compound may be varied in situ thus altering the percentage of fibers within the reinforced compound, hence ultimately in the structural component.
The disposition of fiber within the structural component may also be designated, as alluded to above, by the maneuvering capabilities of the apparatus of the present invention thus allowing, for example, continuous elongated fibers to be positioned congruent with one another or varied creating a cross-weaving fiber arrangement as desired.
Further, the integrity of the fiber is preserved prior to compounding by not introducing the fiber into the extruder barrel at an upstream location thus not subjecting the fiber to damage within the extruder due to the mechanical shear forces induced by the rotation of the screw within the barrel and the heat resulting therefrom. Correspondingly, the extruder is spared the undesired wear associated with introducing the fiber directly into the extruder. Also, the fiber may be maintained at a predetermined tension throughout the compounding process enhancing the alignment of fiber and facilitating the wet-out process while ensuring consistent and uniform distribution of the fiber, thus maximizing the structural benefits of the final reinforced molded component.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.